Dose-based end-pointing for low-kv fib milling in tem sample preparation

ABSTRACT

A method, system, and computer-readable medium for forming transmission electron microscopy sample lamellae using a focused ion beam including directing a high energy focused ion beam toward a bulk volume of material; milling away the unwanted volume of material to produce an unfinished sample lamella with one or more exposed faces having a damage layer; characterizing the removal rate of the focused ion beam; subsequent to characterizing the removal rate, directing a low energy focused ion beam toward the unfinished sample lamella for a predetermined milling time to deliver a specified dose of ions per area from the low energy focused ion beam; and milling the unfinished sample lamella with the low energy focused ion beam to remove at least a portion of the damage layer to produce the finished sample lamella including at least a portion of the feature of interest.

The present application is a continuation of U.S. patent applicationSer. No. 13/600,843, filed Aug. 31, 2012, which is hereby incorporatedby reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to the preparation of samples fortransmission electron microscopy (TEM) or scanning transmission electronmicroscopy (STEM), and in particular the use of charged particle beamsin preparing TEM or STEM samples.

BACKGROUND OF THE INVENTION

Transmission electron microscopy (TEM) enables observers to form imagesof extremely small features, on the order of nanometers to fractions ofAngstroms. TEM also allows analysis of the internal structure of asample. In a TEM, a broad beam of electrons impacts the sample, andelectrons that are transmitted through the sample are focused to form animage of the sample. The sample must be sufficiently thin to allow manyof the electrons in the primary beam to travel though the sample andexit on the opposite site.

A related type of microscopy, scanning transmission electron microscopy(STEM) has similar requirements and capabilities.

A thin TEM sample cut from a bulk sample material is known as a“lamella”. Lamellae are typically less than 100 nanometers (nm) thick,but for some applications a lamella must be considerably thinner. Withadvanced semiconductor fabrication processes at 30 nm and smaller, alamella often needs to be less than 20 nm in thickness in order to avoidoverlap among small scale structures. Thickness variations in the samplecan result in lamella bending, overmilling, or other catastrophicdefects. For such thin samples, lamella preparation is a critical stepin TEM analysis that significantly determines the quality of structuralcharacterization and analysis of the smallest and most criticalstructures.

Prior art methods for TEM lamella preparation typically make use ofvarious milling operations performed by a focused ion beam (FIB) system.Such milling operations include cleaning cross-sections, regularcross-sections, and box mills placed in a manner such that the placementof the mill pattern determines final location of an edge of the lamella.The accuracy of lamella thickness and the final lamella center locationwere based on the accuracy of the placement of these FIB millingoperations. In an automated work flow, all milling is typicallyperformed with respect to some feature or fiducial on the top surface ofthe substrate from which the TEM sample lamella is to be milled.

A known issue involving the production of lamellae in crystallinematerials (silicon is a commercially important example) is that a highenergy focused ion beam (e.g., 30 kiloelectron volts (keV)) produces asubstantial damage layer in the final lamella. The damage layer iscaused, for example, by high energy ions disrupting the crystallinelattice of the sample. A known solution is to perform some finalprocessing steps at lower FIB energy, typically 2 keV to 5 keV, but ingeneral not more than 8 keV. These lower FIB energy processing steps areoften referred to as “damage removal” steps. In some cases, even lowerlanding energies (less than 2 keV) are used. In general, the lower thelanding energy, the less the disruption of the crystalline lattice andthe resulting damage layer thickness decreases with lowered landingenergy.

Low landing energy operation is also sometimes referred to as low-kVoperation because, if the sample is at ground potential, then thelanding energy is directly related to the high voltage potential on theion source tip.

A problem associated with low-kV (kilovolt) damage removal procedures isthat FIB resolution and probe characteristics are substantially degradedat low-kV. The FIB resolution and probe characteristics are degradedbecause chromatic aberrations typically result in substantialdegradations in probe forming performance at low-kV.

This means all steps involving imaging, such as steps used to place thefinal low-kV damage removal mills, have degraded capability. Typicallylamellae are created in automated processes where the placement oflow-kV mills critically impacts the final cut placement and thicknessprecision. The end result is that the control of the placement of edgesis much better at 30 kV then it is at low kV, and the process of damageremoval introduces undesirably large amounts of uncertainty intothickness and position of the final lamella.

SUMMARY OF THE INVENTION

An exemplary embodiment of the present invention comprises a method forforming a transmission electron microscopy sample lamella using afocused ion beam. The method includes directing a high energy focusedion beam toward a bulk volume of material, the bulk volume of materialincluding a feature of interest and an unwanted volume of material, tomill away the unwanted volume of material; milling away the unwantedvolume of material with the high energy focused ion beam to produce anunfinished sample lamella having a thickness that is greater than thedesired thickness of a finished sample lamella, one or more exposedfaces of the unfinished sample lamella including a damage layer;characterizing the material removal rate of a low energy focused ionbeam at a specified time prior to directing the low energy focused ionbeam toward the unfinished sample lamella; subsequent to characterizingthe material removal rate of the low energy focused ion beam, directingthe low energy focused ion beam toward one or more of the exposed facesof the unfinished sample lamella for a predetermined pattern millingtime to deliver a specified dose of ions per area from the low energyfocused ion beam; and milling one or more of the exposed faces of theunfinished sample lamella with the low energy focused ion beam to removeat least a portion of the damage layer, thereby producing the finishedsample lamella including at least a portion of the feature of interest.

Another exemplary embodiment of the present invention comprises a systemfor forming a transmission electron microscopy sample lamella. Thesystem includes a focused ion beam column; a sample stage; a sampledisposed on or within the sample stage; a programmable controller, thecontroller causing the system to automatically: direct a high energyfocused ion beam toward a bulk volume of material, the bulk volume ofmaterial including a feature of interest and an unwanted volume ofmaterial, to mill away the unwanted volume of material; mill away theunwanted volume of material with the high energy focused ion beam toproduce an unfinished sample lamella having a thickness that is greaterthan the desired thickness of a finished sample lamella, one or moreexposed faces of the unfinished sample lamella including a damage layer;characterize the material removal rate of a low energy focused ion beamat a specified time prior to directing the low energy focused ion beamtoward the unfinished sample lamella; subsequent to characterizing thematerial removal rate of the focused ion beam, direct the low energyfocused ion beam toward one or more of the exposed faces of theunfinished sample lamella for a predetermined pattern milling time todeliver a specified dose of ions per area from the low energy focusedion beam; and mill one or more exposed faces of the unfinished samplelamella with the low energy focused ion beam to remove at least aportion of the damage layer, thereby producing the finished samplelamella including at least a portion of the feature of interest.

Another exemplary embodiment of the present invention comprises anon-transitory computer-readable medium encoded with a computer programfor automatically forming a transmission electron microscopy samplelamella, the computer program comprising computer instructions that,when executed by a computer processor, cause a computer controlling afocused ion beam system to direct a high energy focused ion beam towarda bulk volume of material, the bulk volume of material including afeature of interest and an unwanted volume of material, to mill away theunwanted volume of material; mill away the unwanted volume of materialwith the high energy focused ion beam to produce an unfinished samplelamella having a thickness that is greater than the desired thickness ofa finished sample lamella, one or more exposed faces of the unfinishedsample lamella including a damage layer; characterize the materialremoval rate of the low energy focused ion beam at a specified timeprior to directing the low energy focused ion beam toward the unfinishedsample lamella; subsequent to characterizing the material removal rateof the low energy focused ion beam, direct the low energy focused ionbeam toward one or more of the exposed faces of the unfinished samplelamella for a predetermined pattern milling time to deliver a specifieddose of ions per area from the low energy focused ion beam; and mill theone or more exposed faces of the unfinished sample lamella with the lowenergy focused ion beam to remove at least a portion of the damagelayer, thereby producing the finished sample lamella including at leasta portion of the feature of interest.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter. It should be appreciated by those skilled in the art thatthe conception and specific embodiments disclosed may be readilyutilized as a basis for modifying or designing other structures forcarrying out the same purposes of the present invention. It should alsobe realized by those skilled in the art that such equivalentconstructions do not depart from the spirit and scope of the inventionas set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more thorough understanding of the present invention, andadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 shows a lamella 104 cut from a sample substrate 102 including afeature to be analyzed prior to the lamella being removed from thesample substrate;

FIG. 2 shows a top view of lamella 104 cut from sample substrate 102prior to the lamella being removed from the sample substrate;

FIG. 3 shows a top view of one end of unfinished lamella 104, includingdamage layers 302 a-b and the location of final lamella 304;

FIG. 4 shows a top view of unfinished lamella 104 and the placement ofexemplary low-kV milling area 402 to perform a final mill;

FIG. 5 shows a side view of unfinished lamella 104 during low-kVmilling;

FIG. 6 shows a top down view of finished lamella 304;

FIG. 7 shows one embodiment of an exemplary charged particle beam system702 that is equipped to carry out embodiments of the present invention;and

FIG. 8 shows a flowchart depicting a low-kV method of forming a TEMsample lamella using dose-based end-pointing in accordance with one ormore embodiments of the present invention.

FIG. 9 shows a flowchart depicting a method for using feedback fromprocessed sites to improve performance of the final milling.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments of the present invention are directed to methods and systemsfor forming sample lamellae utilizing low energy charged particle beammilling with dose-based end-pointing. Some embodiments of the presentinvention include fully automated methods and systems for forming samplelamellae utilizing low energy charged particle beam milling withdose-based end-pointing. Some embodiments of the present inventioninclude methods and systems for forming sample lamellae utilizingautomated steps and manual steps performed by an operator of theinstrument. In at least one embodiment, the charged particle beam is afocused ion beam and the sample lamella is a transmission electronmicroscopy (TEM) lamella and/or scanning transmission electronmicroscopy (STEM) lamella. Standard high-kV (˜30 kV) milling isperformed with good accuracy of lamella edge placement. High-kV millsare placed with standard procedures, typically making use of fiducialson the substrate surface near a feature of interest, at least a portionof which is to be included in the finished sample lamella. High kVlamella thickness and placement is determined by fiducial accuracyissues as is known in the art.

After an accurate high-kV milling is finished, the lamella is properlylocated and is milled to a precisely controlled thickness. However thethickness of the lamella is slightly larger than the intended finalthickness so that the damage layer caused by the high-kV milling can beremoved with a final mill. In a preferred embodiment, the lamella isthicker than the desired final thickness of the lamella by a lengthapproximately twice that of the penetration depth of the beam being usedfor high-kV milling. In the example of a 30 kV Gallium ion beam, thatpenetration depth is approximately 30 nm.

A dose-based approach is used to remove the damage layer. A final low-kVmill is performed with a pattern placed in a manner such that theaccuracy of the placement of the mill is not important, only the finaldose is important. In a preferred embodiment, a “box” mill is performedwith size larger than the lamella area. In some cases the lamella beingcreated will be extracted from a larger sample, such as a wafer, orpiece of a wafer, in which there are structures near the top surface ofthe sample. In order to minimize certain types of undesirable artifactsassociated with milling through these structures, it is desirable tohave a significant angle between the incident beam and a ray normal tothe top surface of the sample in the plane of the lamella face. It istypically desirable for this angle to be greater than thirty (30)degrees. In one embodiment, the FIB is directed at an angle that issubstantially 45 degrees relative to the top surface of the sample.Other embodiments include an additional rotation of the FIB about theaxis normal to the sample surface. Yet another embodiment includescompound angles involving both a substantial azimuthal and substantialpolar angle of the ion beam relative to surface normal. These angles canbe achieved in a variety of manner sometimes involving multi-axisstages, and sometimes involving movable columns or columns with thedesired fixed orientation.

An additional aspect of embodiments of the present invention includesproviding an accurate characterization of the removal rate of materialon the lamella face and performing a time-controlled low-kV mill basedon the characterization of the material removal rate. In someembodiments, the removal rate of material on the lamella face ischaracterized by determining an accurate dose of the charged particlesdelivered by the low-kV beam based on accurate calibration of the beamcurrent. The accurate calibration of the beam current accounts forvariations in beam current that occur over time. The delivered dose ofcharged particles is a function of the scanned area, the time spentscanning, and the beam current used while scanning. Typically thelargest uncertainty is associated with the ion beam current. To providegood control of the dose, the beam current is measured at regularintervals or within a predetermined time proximity to the dose-basedmill. This measurement can be performed at any specified time prior tothe lamella preparation. For example, the specified time for themeasurement can be a fraction of a second prior to the start of a mill,a minute prior to the start of a mill, once per day, once per week, onceper wafer, etc. Methods of calibrating the FIB current generally consistof directing the ion beam into a collection electrode through which anaccurate current measurement can be made by means of accurate currentmeasurement electronics. This measurement can be done inside the ioncolumn, or at a location exterior to the ion column.

Control of removed material is not dependent on low-kV imaging orpattern recognition, but purely on accurate control of the final dose.Careful measurement of FIB beam current immediately prior to milling(e.g., to no more than 1% error in beam current) and careful control ofmill timing (e.g., to no more than 1% timing error) leads to an error inthe amount of material removed from the unfinished lamella that is nomore than 2%. In an exemplary case where 30 nm of material is removedfrom each side of the unfinished lamella, control of the milling errorto no more than 2% limits the errors introduced by the low-KV mill toless than a nanometer. The numbers used in this paragraph are purely forillustrative purpose.

Calibrating the beam current is not the only way to characterize thematerial removal rate. In some embodiments of the present invention,characterization of the removal rate is performed by experimentallymeasuring the material removal rate at low-kV for the beam conditionsbeing used. For example, a low-kV mill can be experimentally performedto determine that, for a given set of beam conditions, 1 nm of materialis being removed from the lamella face per minute. The experimentallymeasured characterization of the material removal rate can be performedat any specified time prior to the lamella preparation. For example, thespecified time for the measurement can be a fraction of a second priorto the start of a mill, a minute prior to the start of a mill, once perday, once per week, once per wafer, etc.

In another preferred embodiment of the invention, feedback fromprocessed sites may be combined with the timing and dose control or maybe used to determine suitable values for timing and dose. For example,during the production of one or more lamellae according to embodimentsof the present invention, an accurate record is made of how much dosewas delivered to each lamella during its production based on the carefulcontrol of mill timing and monitoring of the actual beam current duringproduction of the lamellae. The lamellae are subsequently reviewed todetermine how well the processing achieved its target objective and thisinformation is feed back into the system to adjust the target dose forthe production of additional lamellae. This review may be performed byany practical method, including a SEM image in the system inside thetool used for the low kV thinning or information collected from the TEMor STEM system that produces the final image of the lamella. A machinevision algorithm can be used to measure the characteristics of thelamella to calculate an adjustment factor to be used in determiningsubsequent doses.

In another example, the system processes multiple samples through thecompletion of the high-kV operations. Then the system applies the low-kVthinning operation to a subset of the samples and collects the samepieces of information as the above example. The tool collects SEM imagesof the completed lamella and a person reviews the result for quality.The person can indicate what scale factor should be delivered to theremaining sites. Alternatively, a machine vision algorithm can be usedto measure the characteristics of the lamella to calculate an adjustmentfactor to be used in determining subsequent doses. When subsequentlamellae are produced, the information based on the SEM image review iscombined with the new actual measured beam current on the preparationtool to reduce the actual dose per area delivered to more accuratelyachieve the target sample thickness.

Embodiments of the present invention are particularly useful for formingTEM sample lamella from a single-crystalline material, such as silicon.Single crystal substrates suffer more damage during high-kV milling thansubstrates that are not formed from a single crystal, such as those usedin data storage applications.

FIG. 1 shows an unfinished lamella 104 cut from a sample substrate 102prior to the lamella being removed from the sample substrate. Unfinishedlamella 104 is formed by milling material away from sample substrate 102in locations surrounding the lamella. The material is milled using acharged particle beam, such as an ion beam, an electron beam, or a laserbeam. In a preferred embodiment, the charged particle beam is a focusedion beam. One or more fiducials (not shown) on sample substrate 102 canbe used to locate the desired lamella location. A high-kV mill isinitially performed with good accuracy of lamella edge placement. Ahigh-kV mill removes material from the sample substrate using a beam ofcharged particles having an energy greater than 8 keV, preferably around30 keV.

FIG. 2 shows a top view of unfinished lamella 104 after performing theinitial high-kV milling. The charged particle beam removes substratematerial from high-kV mill areas 202 a-b to expose vertical lamellafaces 204 a-b. High-kV mill areas 202 a-b are located on either side oflamella 104. One or more fiducials (not shown) may be used determine thelocation of high-kV mill areas 202 a-b on the sample substrate. As aconsequence of the high-kV milling used to remove the material fromhigh-kV mill areas 202 a-b, unfinished lamella 104 includes damagelayers that need to be corrected or removed before the lamella can beanalyzed, for example, by a TEM.

FIG. 3 shows a top view of one end of unfinished lamella 104, includingdamage layers 302 a-b and the location of final lamella 304. Damagelayers 302 a-b extend from lamella faces 204 a-b to a certain depthwithin unfinished lamella 104. To allow for the removal of the damagelayers in a subsequent low-kV milling step, the high-kV milling step isperformed so that unfinished lamella 104 has a thickness that is largerthan the intended thickness of final lamella 304. In one or moreembodiments, the high-kV mill is performed so that each side of lamella104 is substantially 30 nanometers (nm) thicker than the intendedthickness for that side of finished lamella 304.

Damage layers 302 a-b result from the initial high-kV milling ofunfinished lamella 104. Milling with a high energy charged particle beamhas the benefit of higher mill rates and more accurate beam placementbecause chromatic aberration is reduced. But higher energy particlesalso cause damage to the sample substrate, producing damage layers 302a-b. For example, high-kV milling of a silicon crystal substrate with afocused ion beam can cause unwanted damage to the crystal lattice.Therefore, embodiments of the present invention include performing afinal mill to remove damage layers 302 a-b. The final milling step is alow-kV mill that utilizes dose-based end-pointing.

FIG. 4 shows a top view of unfinished lamella 104 and the placement ofexemplary low-kV milling area 402 to perform a final mill. Low-kV millarea 402 is placed in a manner such that the accuracy of the placementis not important, only the final dose of particles is important.Preferably, a box mill is performed around unfinished lamella 104 with asize larger than the intended final thickness of finished lamella 304.The simplest type of box mill is one in which a serpentine or rasterpattern is traced across a defined geometric shape, typically arectangle, with many repetitions of the pattern being traced out overthe duration of the mill. For the purpose of this invention, the keyconcept is that a box mill is differentiated from a cleaning crosssection mill in that mills of the style of a cleaning cross section havea location of milling that is slowly progressing. For the purpose ofthis invention the exact details of the box mill pattern are notimportant, for example pattern could be traced out across a defined areathat is substantially circular. Low-kV mill area 402 includessubstantially all of damage layers 302 a-b so that upon the completionof the low-kV milling operation substantially all of the damage layers302 a-b are removed from finished lamella 304. Because low-kV milling isused instead of high-kV milling, damage to finished lamella 304 iseliminated or significantly reduced compared to the damage to unfinishedlamella 304 caused by the high-kV milling.

FIG. 5 shows a side view of unfinished lamella 104 during low-kVmilling. Performance can be improved by operating the charged particlebeam 502 in a manner that is not substantially “top-down”. In some caseslamella 104 will be extracted from a larger sample, such as a wafer, orpiece of a wafer, in which there are structures near the top surface ofthe sample 102. In order to minimize certain types of undesirableartifacts associated with milling through these structures, it isdesirable to have a significant angle (θ) between the incident beam anda ray 504 normal to the top surface of the sample in the plane of theface of lamella 104. It is typically desirable for angle θ to be greaterthan thirty (30) degrees. In one embodiment, the FIB is directed at anangle θ that is substantially 45 degrees relative to the top surface ofthe sample.

FIG. 6 shows a top down view of finished lamella 304. Substantially allof damage layers 302 a-b are removed from finished lamella 304 byperforming low-kV milling. Finished lamella 304 may still be attached atits base to sample substrate 102. Further processing, such asundercutting, may be performed to separate lamella 304 from the samplesubstrate for analysis in another instrument.

FIG. 8 shows a flowchart depicting a low-kV method of forming a TEMsample lamella using dose-based end-pointing in accordance with one ormore embodiments of the present invention. The method starts at 802 andproceeds to step 804, where a high energy charged particle beam isdirected at sample substrate 102. The high energy charged particle beamis directed at sample substrate 102 to perform a high-kV millingoperation. The high-kV milling operation uses the high energy beam toremove an unwanted volume of bulk substrate material from high-kV millareas 202 a-b to expose vertical lamella faces 204 a-b of unfinishedlamella 104 (step 806). The high energy charged particle beam haslanding energies greater than 8 keV, and preferably around 30 keV.

Prior to performing damage removal with a low energy beam, the materialremoval rate of the low energy charged particle beam is characterized(step 808). The characterization of the removal rate can be accomplishedby accurately measuring the beam current to determine a calibrated beamcurrent, by experimentally measuring the material removal rate of thelow energy beam, or any other suitable method for characterizing theremoval rate of the low energy beam. If the removal rate ischaracterized by measuring the bean current, then the dose of chargedparticles is controlled by accurate calibration of the beam current toaccount for day to day (or even more frequent) variation in beamcurrent. In one embodiment of the present invention, the beam currentmeasurement of step 808 is performed immediately prior to performing thelow-kV milling steps. Alternatively, the beam current measurement ofstep 808 can be performed at regularly space time intervals. Methods ofcalibrating the beam current include, but are not limited to, blankedbeam measurements with a calibrated picoammeter and measurements of beamcurrent using a stage Faraday cup.

After milling unfinished lamella 104 with a high energy charged particlebeam and characterizing the material removal rate, a low energy chargedparticle beam is directed toward unfinished lamella 104 (step 810). Thelow energy particle beam has landing energies of less than 8 keV, andpreferably between 2 keV and 5 keV. The low energy charged particle beamis directed at unfinished lamella 104 to perform a low-kV millingoperation. The low-kV milling operation uses the low energy beam toremove damage layers 302 a-b (step 812). The low-kV milling step isprecisely timed so that a predetermined dose of charged particles isdelivered based on the milling time and the material removal rate (step814). When the predetermined dose of charged particles has beendelivered, the method stops at terminator 816. The finished samplelamella includes at least a portion of the feature of interest.

Control of the removed material is not dependent upon low-kV imaging orpattern recognition, but purely on the accurate control of the finaldose. The final dose in controlled by accurate characterization of thematerial removal rate at a specified time prior to milling and carefulcontrol of the timing of the mill. In a preferred embodiment, timingerror is controlled to less than one percent (1%) and error in beamcurrent is controlled to less than one percent (1%). This leads to anerror in material removed that is less than two percent (2%). Forexample, if thirty nanometers (30 nm) of material is removed on eachside of unfinished lamella 104, control of material removed to less than2% allows for sub-nanometer errors introduced by the low-kV millingoperation.

Embodiments of the present invention are described herein with respectto forming TEM lamellae. One skilled in the art will recognize thatembodiments of the present invention are not only limited to forming TEMlamellae, but also apply to forming other types of lamellae, such asSTEM lamellae.

In preferred embodiments, the method of FIG. 8 is fully automated andthe final end-pointing of finished lamella 304 does not require anyhuman interaction, in particular any human visual interaction. The beamcurrent is measured immediately prior to the low-kV milling steps andthe low-kV milling time is adjusted based on the most recent beamcurrent calibration to give extremely accurate dose control. The millingpattern is placed in a manner so as to “over-expose” the region aroundthe region of interest such that the position of the placement haslittle or no impact to the material removed from the region of interest.

FIG. 9 shows a flowchart depicting a method for using feedback fromprocessed sites to improve performance of the final milling. In thisembodiment of the invention, feedback from processed sites may becombined with the timing and dose control or may be used to determinesuitable values for timing and dose. The method begins start block 902and proceeds to step 904. A first set of one or more lamellae areproduced according the method shown in FIG. 8. An accurate record ismade of how much dose was delivered to each lamella during step 904based on the careful control of mill timing and monitoring of the actualbeam current during production of the lamellae. The lamellae aresubsequently reviewed to determine how well the processing achieved itstarget objective. One or more characteristics of the lamellae aremeasured either while the lamellae are being milled or at a point intime after the milling of the lamellae is complete (step 906). The oneor more characteristics of the lamellae are recorded (step 908), such aslamella thickness, the size of the remaining damage layer, an erroroffset in mill placement, etc. A second specified dose of ions per areafor the low energy focused ion beam is calculated based on thedifference between an intended lamella characteristic and the one ormore measured characteristics of the first set of lamellae (step 910).This information is feed back into the system to adjust the target dosefor the production of additional lamellae. This review may be performedby any practical method, including a SEM image in the system inside thetool used for the low kV thinning or information collected from the TEMor STEM system that produces the final image of the lamella. A secondset of one or more sample lamellae is produced using a second patternmilling time to deliver the second specified dose of ions per area forthe low energy focused ion beam (step 912). The process ends atterminator 914. This feedback process can be applied multiple times toimprove the accuracy of the milling of the finished sample lamellae.

For example, using embodiments of the methods of FIGS. 8 and 9, thesystem producing the lamella collects the following pieces ofinformation: (1) the sample was 95.0 nm thick prior to the start of thelow-kV mill, (2) the low-kV mill operated at an acceleration voltage of2 kV, (3) the low-kV mill applied to each side delivered to a targetregion with an area of 9.00 μm² for a duration of 35.7 seconds, and (4)the measured beam current at the time of the low-kV mill was 85.4 pA.The TEM system imaging the lamella determines that the sample thicknesswas 4% less than the optimal thickness. When subsequent lamellae areproduced, the information from the TEM system is combined with the newactual measured beam current on the preparation tool to reduce theactual dose per area delivered to more accurately achieve the targetsample thickness.

In another example, the system processes multiple samples through thecompletion of the high-kV operations. Then the system applies the low-kVthinning operation to a subset of the samples and collects the samepieces of information as the above example. The tool collects SEM imagesof the completed lamella and a person reviews the result for quality.The person can indicate what scale factor should be delivered to theremaining sites. When subsequent lamellae are produced, the informationbased on the SEM image review is combined with the new actual measuredbeam current on the preparation tool to reduce the actual dose per areadelivered to more accurately achieve the target sample thickness.

FIG. 7 depicts one embodiment of an exemplary dual beam SEM/FIB system702 that is equipped to carry out embodiments of the present invention.The present invention does not require a dual beam system, and can bereadily used with any charged particle beams system, including a singleFIB system. The dual-beam system is described here for exemplarypurposes only. Preparation and analysis of such a TEM sample can beperformed in a dual beam electron beam/focused ion beam system such asthe one now described. Suitable charged particle beam systems arecommercially available, for example, from FEI Company, Hillsboro, Oreg.,the assignee of the present application. While an example of suitablehardware is provided below, the invention is not limited to beingimplemented in any particular type of hardware.

Dual beam system 702 has a vertically mounted electron beam column 704and a focused ion beam (FIB) column 706 mounted at an angle ofapproximately 52 degrees from the vertical on an evacuable specimenchamber 708. The specimen chamber may be evacuated by pump system 709,which typically includes one or more, or a combination of, aturbo-molecular pump, oil diffusion pumps, ion getter pumps, scrollpumps, or other known pumping means.

The electron beam column 704 includes an electron source 710, such as aSchottky emitter or a cold field emitter, for producing electrons, andelectron-optical lenses 712 and 714 forming a finely focused beam ofelectrons 716. Electron source 710 is typically maintained at anelectrical potential of between 500 V and 30 kV above the electricalpotential of a work piece 718, which is typically maintained at groundpotential.

Thus, electrons impact the work piece 718 at landing energies ofapproximately 500 eV to 30 keV. A negative electrical potential can beapplied to the work piece to reduce the landing energy of the electrons,which reduces the interaction volume of the electrons with the workpiece surface, thereby reducing the size of the nucleation site. Workpiece 718 may comprise, for example, a semiconductor device,microelectromechanical system (MEMS), data storage device, or a sampleof material being analyzed for its material characteristics orcomposition. The impact point of the beam of electrons 716 can bepositioned on and scanned over the surface of a work piece 718 by meansof deflection coils 720. Operation of lenses 712 and 714 and deflectioncoils 720 is controlled by scanning electron microscope power supply andcontrol unit 722. Lenses and deflection unit may use electric fields,magnetic fields, or a combination thereof.

Work piece 718 is on movable stage 724 within specimen chamber 708.Stage 724 can preferably move in a horizontal plane (X-axis and Y-axis)and vertically (Z-axis) and can tilt approximately sixty (60) degreesand rotate about the Z-axis. A door 727 can be opened for inserting workpiece 718 onto X-Y-Z stage 724 and also for servicing an internal gassupply reservoir (not shown), if one is used. The door is interlocked sothat it cannot be opened if specimen chamber 708 is evacuated.

Mounted on the vacuum chamber are one or more gas injection systems(GIS) 730. Each GIS may comprise a reservoir (not shown) for holding theprecursor or activation materials and a needle 732 for directing the gasto the surface of the work piece. Each GIS further comprises means 734for regulating the supply of precursor material to the work piece. Inthis example the regulating means are depicted as an adjustable valve,but the regulating means could also comprise, for example, a regulatedheater for heating the precursor material to control its vapor pressure.

When the electrons in the electron beam 716 strike work piece 718,secondary electrons, backscattered electrons, and Auger electrons areemitted and can be detected to form an image or to determine informationabout the work piece. Secondary electrons, for example, are detected bysecondary electron detector 736, such as an Everhart-Thornley detector,or a semiconductor detector device capable of detecting low energyelectrons. STEM detector 762, located beneath the TEM sample holder 761and the stage 724, can collect electrons that are transmitted through asample mounted on the TEM sample holder. Signals from the detectors 736,762 are provided to a programmable system controller 738. Saidcontroller 738 also controls the deflector signals, lenses, electronsource, GIS, stage and pump, and other items of the instrument. Monitor740 is used to display user controls and an image of the work pieceusing the signal. Said controller 738 may comprise a programmablegeneral purpose computer including tangible, non-transitorycomputer-readable medium, the memory being encoded with computerinstructions that, when executed by a processor of the computer causesthe computer to automatically perform embodiments of the presentinvention, such as the method depicted in FIG. 8.

The chamber 708 is evacuated by pump system 709 under the control ofvacuum controller 741. The vacuum system provides within chamber 708 avacuum of approximately 7×10-6 mbar. When a suitable precursor oractivator gas is introduced onto the sample surface, the chamberbackground pressure may rise, typically to about 5×10-5 mbar.

Focused ion beam column 706 comprises an upper neck portion 744 withinwhich are located an ion source 746 and a focusing column 748 includingextractor electrode 750 and an electrostatic optical system including anobjective lens 751. Ion source 746 may comprise a liquid metal galliumion source, a plasma ion source, a liquid metal alloy source, or anyother type of ion source. The axis of focusing column 748 can beoriented at a non-zero angle from the axis of the electron column. Anion beam 752 passes from ion source 746 through focusing column 748 andbetween electrostatic deflectors 754 toward work piece 718.

FIB power supply and control unit 756 provides an electrical potentialat ion source 746. Ion source 746 is typically maintained at anelectrical potential of between 1 kV and 60 kV above the electricalpotential of the work piece, which is typically maintained at groundpotential. Thus, ions impact the work piece at landing energies ofapproximately 1 keV to 60 keV. FIB power supply and control unit 756 iscoupled to deflection plates 754 which can cause the ion beam to traceout a corresponding pattern on the upper surface of work piece 718. Insome systems, the deflection plates are placed before the final lens, asis well known in the art. Beam blanking electrodes (not shown) withinion beam focusing column 748 cause ion beam 752 to impact onto blankingaperture (not shown) instead of work piece 718 when a FIB power supplyand control unit 756 applies a blanking voltage to the blankingelectrode.

The ion source 746 typically provides a beam of singly charged positivegallium ions that can be focused into a sub one-tenth micrometer widebeam at work piece 718 for modifying the work piece 718 by ion milling,enhanced etch, material deposition, or for imaging the work piece 718.

A micromanipulator 757, such as the AutoProbe 200™ from Omniprobe, Inc.,Dallas, Tex., or the Model MM3A from Kleindiek Nanotechnik, Reutlingen,Germany, can precisely move objects within the vacuum chamber.Micromanipulator 757 may comprise precision electric motors 758positioned outside the vacuum chamber to provide X, Y, Z, and thetacontrol of a portion 759 positioned within the vacuum chamber. Themicromanipulator 757 can be fitted with different end effectors formanipulating small objects. In the embodiments described herein, the endeffector is a thin probe 760. As is known in the prior art, amicromanipulator (or microprobe) can be used to transfer a TEM sample(which has been freed from a substrate, typically by an ion beam) to aTEM sample holder 761 for analysis.

System controller 738 controls the operations of the various parts ofdual beam system 702. Through system controller 738, a user can causeion beam 752 or electron beam 716 to be scanned in a desired mannerthrough commands entered into a conventional user interface (not shown).Alternatively, system controller 738 may control dual beam system 702 inaccordance with programmed instructions. FIG. 7 is a schematicrepresentation, which does not include all the elements of a typicaldual beam system and which does not reflect the actual appearance andsize of, or the relationship between, all the elements.

Although the description of the present invention above is mainlydirected at methods of preparing ultra-thin TEM samples, it should berecognized that an apparatus performing the operation of such a methodwould further be within the scope of the present invention. Further, itshould be recognized that embodiments of the present invention can beimplemented via computer hardware, a combination of both hardware andsoftware, or by computer instructions stored in a non-transitorycomputer-readable memory. The methods can be implemented in computerprograms using standard programming techniques—including anon-transitory computer-readable storage medium configured with acomputer program, where the storage medium so configured causes acomputer to operate in a specific and predefined manner—according to themethods and figures described in this Specification. Each program may beimplemented in a high level procedural or object oriented programminglanguage to communicate with a computer system. However, the programscan be implemented in assembly or machine language, if desired. In anycase, the language can be a compiled or interpreted language. Moreover,the program can run on dedicated integrated circuits programmed for thatpurpose.

Further, methodologies may be implemented in any type of computingplatform, including but not limited to, personal computers,mini-computers, main-frames, workstations, networked or distributedcomputing environments, computer platforms separate, integral to, or incommunication with charged particle tools or other imaging devices, andthe like. Aspects of the present invention may be implemented in machinereadable code stored on a storage medium or device, whether removable orintegral to the computing platform, such as a hard disc, optical readand/or write storage mediums, RAM, ROM, and the like, so that it isreadable by a programmable computer, for configuring and operating thecomputer when the storage media or device is read by the computer toperform the procedures described herein. Moreover, machine-readablecode, or portions thereof, may be transmitted over a wired or wirelessnetwork. The invention described herein includes these and other varioustypes of computer-readable storage media when such media containinstructions or programs for implementing the steps described above inconjunction with a microprocessor or other data processor. The inventionalso includes the computer itself when programmed according to themethods and techniques described herein.

Computer programs can be applied to input data to perform the functionsdescribed herein and thereby transform the input data to generate outputdata. The output information is applied to one or more output devicessuch as a display monitor. In preferred embodiments of the presentinvention, the transformed data represents physical and tangibleobjects, including producing a particular visual depiction of thephysical and tangible objects on a display.

Preferred embodiments of the present invention also make use of aparticle beam apparatus, such as a FIB or SEM, in order to image asample using a beam of particles. Such particles used to image a sampleinherently interact with the sample resulting in some degree of physicaltransformation. Further, throughout the present specification,discussions utilizing terms such as “calculating,” “determining,”“measuring,” “generating,” “detecting,” “forming,” or the like, alsorefer to the action and processes of a computer system, or similarelectronic device, that manipulates and transforms data represented asphysical quantities within the computer system into other data similarlyrepresented as physical quantities within the computer system or otherinformation storage, transmission or display devices.

The invention has broad applicability and can provide many benefits asdescribed and shown in the examples above. The embodiments will varygreatly depending upon the specific application, and not everyembodiment will provide all of the benefits and meet all of theobjectives that are achievable by the invention. Particle beam systemssuitable for carrying out the present invention are commerciallyavailable, for example, from FEI Company, the assignee of the presentapplication.

Although much of the previous description is directed at semiconductorwafers, the invention could be applied to any suitable substrate orsurface. Further, the present invention could be applied to samples thatare thinned in the vacuum chamber but removed from the substrate outsidethe vacuum chamber (ex-situ-type samples) or to samples extracted fromthe substrate and thinned after mounting on a TEM grid inside the vacuumchamber (in-situ-type samples). Whenever the terms “automatic,”“automated,” or similar terms are used herein, those terms will beunderstood to include manual initiation of the automatic or automatedprocess or step. In the following discussion and in the claims, theterms “including” and “comprising” are used in an open-ended fashion,and thus should be interpreted to mean “including, but not limited to .. . .” The term “integrated circuit” refers to a set of electroniccomponents and their interconnections (internal electrical circuitelements, collectively) that are patterned on the surface of amicrochip. The term “semiconductor device” refers generically to anintegrated circuit (IC), which may be integral to a semiconductor wafer,singulated from a wafer, or packaged for use on a circuit board. Theterm “FIB” or “focused ion beam” is used herein to refer to anycollimated ion beam, including a beam focused by ion optics and shapedion beams.

To the extent that any term is not specially defined in thisspecification, the intent is that the term is to be given its plain andordinary meaning. The accompanying drawings are intended to aid inunderstanding the present invention and, unless otherwise indicated, arenot drawn to scale.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. Moreover, thescope of the present application is not intended to be limited to theparticular embodiments of the process, machine, manufacture, compositionof matter, means, methods and steps described in the specification. Asone of ordinary skill in the art will readily appreciate from thedisclosure of the present invention, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the present invention.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps.

1. A method for forming a lamella using ion beam, comprising: millingone or both sides of an unfinished lamella using a high energy ion beamto expose one or two faces of the unfinished lamella, each of the one ortwo faces of the unfinished lamella including a damage layer; millingthe one or two faces of the unfinished lamella to remove at least aportion of the damage layer using a low energy ion beam to produce atleast a partially finished lamella; and terminating milling the one ortwo faces of the unfinished lamella responsive to delivering apredetermined dose of ions per area by the low energy ion beam to the atleast a portion of the damage layer.
 2. The method of claim 1, furthercomprising: measuring a material removal rate of the low energy ionbeam; and determining a milling time based on the measured materialremoval rate; wherein terminating milling the one or two faces of theunfinished lamella responsive to delivering the predetermined doseincludes terminating milling the one or two faces of the unfinishedlamella after the milling time.
 3. The method of claim 1, furthercomprising: measuring a beam current of the low energy ion beam; anddetermining the predetermined dose based on the measured beam currentbefore milling the one or two faces of the unfinished lamella using thelow energy ion beam.
 4. The method of claim 1, further comprising:recording one or more characteristics of the finished lamella; adjustingthe predetermined dose based on a difference between an intended lamellacharacteristic and the recorded one or more characteristic of thefinished lamella; and producing a second finished lamella based on theadjusted predetermined dose.
 5. The method of claim 4, wherein adjustingthe predetermined dose based on a difference between an intended lamellacharacteristic and the recorded one or more characteristic of thefinished lamella, and producing a second finished lamella based on theadjusted predetermined dose includes: determining a second milling timebased on the difference between the intended lamella characteristic andthe recorded one or more characteristic of the finished lamella; millingone or both sides of a second unfinished lamella using the high energyion beam to expose one or two faces of the second unfinished lamella,each of the one or two faces of the second unfinished lamella includes asecond damage layer; milling the second unfinished lamella to remove theat least a portion of the second damage layer using the low energy ionbeam to produce at least a second partially finished lamella; andterminating milling the second unfinished lamella after the secondmilling time.
 6. The method of claim 1, wherein terminating milling ofthe one or two faces of the unfinished lamella does not depend on lowenergy imaging or pattern recognition.
 7. The method of claim 1, whereinterminating milling of the one or two faces of the unfinished lamelladepends only on the delivery of the predetermined dose.
 8. The method ofclaim 1, wherein milling the one or two faces of the unfinished lamellato remove at least a portion of the damage layer using a low energy ionbeam includes: box milling the one or two faces of the unfinishedlamella using the low energy ion beam, wherein the box mill has with asize that is larger than the one or two faces of the unfinished lamella.9. The method of claim 8, wherein an incidence angle of the low energyion beam toward the one or two faces of the unfinished lamella is lessthan 90 degrees.
 10. The method of claim 1, wherein the high energyfocused ion beam has a landing energy greater than eightkiloelectronvolts (8 keV).
 11. The method of claim 1, wherein the lowenergy focused ion beam has a landing energy less than eightkiloelectronvolts (8 keV).
 12. A method for forming a lamella using ionbeam, comprising: milling a surface on at least one side of anunfinished lamella using a high energy ion beam to expose a face of theunfinished lamella, the face of the unfinished lamella including adamage layer; measuring a beam current of a low energy ion beam;determining a milling time based on the measured beam current; andmilling the exposed face of the unfinished lamella for the milling timeusing the low energy ion beam to produce a finished lamella, wherein atleast a portion of the damage layer is removed by delivering apredetermined dose of ions per area during the milling time.
 13. Themethod of claim 12, wherein milling the exposed face of the unfinishedlamella to remove at least a portion of the damage layer includes boxmilling the exposed face of the unfinished lamella using the low energyion beam, wherein the box mill has with a size that is larger than theexposed face of the unfinished lamella.
 14. The method of claim 12,further comprising: recording one or more characteristics of thefinished lamella; adjusting the milling time based on a differencebetween an intended lamella characteristic and the recorded one or morecharacteristic of the finished lamella; and producing a second finishedlamella by milling a second unfinished lamella using the low energy ionbeam.
 15. A system for forming a lamella, comprising: a focused ion beamcolumn; a sample stage; a sample disposed on or within the sample stage;and a programmable controller programmed with computer instructionsthat, when executed by a computer processor, causes the system toautomatically: mill one or both sides of an unfinished lamella using ahigh energy ion beam to expose one or two faces of the unfinishedlamella, each of the one or two faces of the unfinished lamellaincluding a damage layer; mill the one or two faces of the unfinishedlamella to remove at least a portion of the damage layer using a lowenergy ion beam to produce at least a partially finished lamella; andterminate milling the one or two faces of the unfinished lamellaresponsive to delivering a predetermined dose of ions per area by thelow energy ion beam to at least a portion of the damage layer.
 16. Thesystem of claim 15, wherein the programmable controller is furtherprogrammed to cause the system to automatically: measure a beam currentof the low energy focused ion beam; determine a milling time based onthe measured beam current; and wherein terminate milling the one or twofaces of the unfinished lamella responsive to delivering thepredetermined dose includes terminate milling the one or two faces ofthe unfinished lamella after the milling time.
 17. The system of claim16, wherein the programmable controller is further programmed to causethe system to automatically: measure one or more characteristics of thefinished lamella; adjust the milling time based on a difference betweenan intended lamella characteristic and the recorded one or morecharacteristic of the finished lamella; and produce a second finishedlamella based on the adjusted milling time.
 18. The system of claim 17,wherein the characteristics of the finished lamella includes lamellathickness, size of a remaining damage layer, and an error offset in millplacement.
 19. The system of claim 15, wherein terminate milling of theunfinished lamella does not depend on low energy imaging or patternrecognition.
 20. The system of claim 15, wherein terminate milling ofthe unfinished lamella depends only on the delivery of the predetermineddose.